This present invention relates to paints and printing inks which contain carbon black as the pigment.
The black pigment used in paints and printing inks is predominantly carbon black due to its excellent properties. Apart from use for the production of pure black colors, carbon black is also used for tinting with other pigments, in particular for the production of grey hues by blending carbon black with white pigments such as titanium dioxide and other white pigments.
Pigment blacks are available in a large selection of grades having differing properties. Various processes are used for producing pigment blacks. The most common process is production by oxidative pyrolysis of carbon black feedstocks containing carbon. In this process, the carbon black feedstocks are incompletely combusted at elevated temperatures in the presence of oxygen. This class of carbon black production processes includes, for example, the furnace black process, the gas black process and the lamp black process. The carbon black feedstocks containing carbon which are used are primarily polycyclic aromatic hydrocarbon feedstock oils. The product stream from oxidative pyrolysis consists of an off-gas containing hydrogen and carbon monoxide and finely divided carbon black suspended therein, which is separated from the off-gas in a filter unit.
Production processes by oxidative pyrolysis include the furnace black process, the lamp black process and the gas black process. In the furnace black process, incomplete combustion proceeds in a reactor lined with highly temperature resistant refractory material. A fuel/air mixture is combusted in a precombustion chamber to produce a flame, into which the carbon black feedstock is sprayed or injected. As the carbon black is formed, it is quenched by spraying water into the reactor and separated from the gas stream. The furnace black process allows the production of carbon blacks having a very wide range of technical properties.
Lamp black and gas black processes are important alternatives to the furnace black process. They yield carbon blacks having properties which partially overlap with those technical properties obtainable by the furnace black process, but also allow the production of carbon blacks which cannot be produced using the furnace black process.
The lamp black apparatus consists of a cast iron pan which holds the liquid or optionally molten feedstock and a refractory-lined fume hood. The air gap between the pan and fume hood and the reduced pressure within the system act to control the input of air and thus to influence the properties of the carbon black. As a result of radiant heat input from the fume hood, the feedstock is vaporized and is partially combusted, but mainly converted into carbon black. The carbon black is separated by passing the process gases containing carbon black, once cooled, into a filter.
In the gas black process, the carbon black feedstock is first vaporized in a carrier gas stream containing hydrogen and then combusted in a plurality of small flames beneath a cooled roller. A proportion of the resultant carbon black is deposited on the roller, while another proportion is discharged with the process gases and deposited in a filter.
The important properties for evaluating pigment blacks are jetness (or blackness value) M.sub..gamma. (DIN 55979), tinting strength (according to DIN EN ISO 787/16 and DIN EN ISO 787/24), oil absorption (DIN EN ISO 787/5), volatile matter (DIN 53552), structure, measured as DBP adsorption (DIN 53601, ISO 4656 or ASTM D2414), average primary particle size (by evaluating electron micrographs) and pH value (DIN EN ISO 787/9 or ASTM D1512).
Table 1 shows the range of pigment black properties obtainable by the stated production processes.
TABLE 1 ______________________________________ Furnace Gas Lamp Property black black black ______________________________________ Jetness M.sub.Y 210-270 230-300 200-220 Tinting Strength [%] 60-130 90-130 25-35 IRB3 = 100 Oil absorption [g/100 g] 200-500 220-1100 250-400 Volatile matter [wt. %] 0.5-6.0 4-24 1-2.5 DBP absorption [ml/100 g] 40-200 100-120 Particle size [nm] 10-80 10-30 110-120 Particle size narrow wide distribution pH value 6-10 4-6 6-9 ______________________________________
The most important properties for the selection of pigment blacks are the jetness and structure thereof. Jetness is directly dependent upon particle size. The smaller the particles are, the greater is the jetness of the pigment black. Particle size furthermore influences other properties such as oil absorption and viscosity of the finished paint or printing ink. The structure of the carbon black also influences the viscosity of the finished product and is significant for the production and processing of the paints and printing inks. An elevated structure entails an elevated viscosity at a given pigment content and vice versa.
Important applicational properties of a paint or printing ink are the stability of the carbon black dispersion in the binder system (storage stability), flooding behavior in pigment mixtures and the rheological behavior of the paint or printing ink (viscosity and thixotropy).
The stability of the carbon black dispersion in a binder may, for example, be improved by adding fumed silica. Adding silica may, however, result in an undesirable increase in the viscosity of the product. The elevated viscosity may be offset by adding a greater quantity of solvents. But the greater quantity of solvents is of disadvantage since the pigment loading is reduced and also from environmental considerations.
Paints may exhibit so-called flooding if they contain a mixture of pigments. Flooding is taken to mean the observed phenomenon that, for example, in grey paints the white pigments and the carbon black become separated during paint drying, so distorting the color. This behavior is readily tested by the "rub-out test". In this test, the paint is applied onto a test surface. After partial drying for a short period, one half of the test surface is rubbed out, for example, using a finger. If flooding had brought about separation of the two pigments, this separation is reversed by the mechanical stress during rubbing out and a distinct difference in the color of the two halves is evident. This property may give rise to differing hues on practical use of the paint and should thus be kept as low as possible.
Elevated carbon black concentrations are desired in mill base formulations. This results in economic advantages due to increased throughputs and to environmental advantages due to reduced solvent requirements and the possibility of producing low-solvent, "high-solids" paints.
An object of the present invention is to provide paints and printing inks which are improved with regard to the storage stability, flooding behavior and solvent requirement thereof in comparison with conventional paint formulations. Within the scope of the present invention it is understood that the wording "paints and printing inks" encompasses also non-impact printing inks and toner.